A load giving the least possible losses in relation to its performed work in an electrical system is a load with purely resistive impedance. However, in most cases the impedance of the load an electrical apparatus constitutes almost always contains an inductive or capacitive component, which means that the current consumed from the feeding grid not always will be in phase with the source voltage. Furthermore, many electrical apparatus comprises active components that have a dynamically varying load, i.e. the load varies with time. Such electrical apparatus includes among others the types of apparatus converting electrical power to work, such as cranes or workshop machines. It is obvious that for example a sheet press device has an increased current consumption during active press duty than when the device is only idling. Common for this type of apparatuses is that they comprise a dynamic power supply. Furthermore, in many cases the dynamics are unpredictable, in the sense that the load changes have an uneven or indeed random periodicity, or impedance that varies in strength over time. For such processes, calculation and averaging of the current consumption can as a rule not be utilized in a satisfactory way in order to compensate the load changes. A consequence of such load behaviours is that current transients is formed in the power grid causing to a certain extent increased current consumption depending on energy content, and mainly causing problems for other sensitive equipment connected to the power grid.
In order to solve the problem with the influence of dynamic loads on the power grid so called active filters have been developed, adapted for compensating load variations. An active filter is in principle a microprocessor controlled amplifier which is connected to the power grid, and which is arranged to sense and compensate the load's current consumption in regard to frequencies which would not exist if the load was purely resistive. The active filter at that comprises a main circuit with one or a series of fast switches for each phase. Each such switch preferably comprises a transistor, a diode and a protection circuit in the form of a so-called snubber. These switches are connected to a DC-link that can accumulate electrical energy. The power grid's current provision and the load's current consumption are measured periodically and using pulse width modulation the current direction for the transistor circuits are reversed in such a manner that the current provided by the active filter results in a current drawn from the power grid that is in phase with the voltage and liberated from disturbances. Said switches generally comprise so-called IGBTs, Insulated Gate Bipolar Transistor, but can also be realized with other transistor types, for example MOSFET. Preferably the aforementioned main circuit is connected to each of the three phases of the power grids using inductors. Further, for each phase connection a circuit breaker is preferably arranged between the active filter's main circuit and the phases.
At loads varying unevenly between the phases disturbances may leak to the electrical neutral line. A problem with the thereby generated so-called null currents is that they may become non-neglectable and thereby cause problems in other equipment.
In a simultaneously filed patent application by the inventors for the present application, this is solved by an active filter for compensation of variations in an apparatus's current consumption from a power grid as a cause of changes in the apparatus's electrical load, which filter comprises a switch arranged for reversal of the current direction connected between the neutral line in the power grid and a DC-link.
Each component in an electrical system that is not purely resistive will provide a complex impedance component, and therefore affect the system. The same of course applies to active components that result in dynamic variations of the current consumption. Thus the active filter will of course not only work to compensate undesired components in the system, it can also introduce disturbances.
One purpose with the present invention is to provide an active filter which, in relation to apparatuses and methods according to the state of the art, has an improved capacity to compensate for variations in an apparatus's current consumption from a power grid as a cause of variations in the apparatus's electrical behaviour, and which at the same time decreases the stress the active filter itself brings about on the power grid.